The present invention relates to an image forming apparatus with a multi-beam scanning system. Particularly, this invention relates to an image forming apparatus including a printer and a fax machine, etc., that overcomes a problem in that a formed image is blurred due to positional shifting of dots on each line which occurs when image magnification is not constant over light beams in simultaneous formation of dots on several lines in an image-forming area with light beams. Moreover, the present invention relates to an image forming apparatus that overcomes a problem in that an image-forming area will be shifted due to difference in image magnification on a plane to be exposed to light beams, by detecting scanning light beams on the plane at least two points in the vicinity of the entire plane.
Several techniques have been proposed for image forming apparatus, for example, a plane paper copy machine (called PPC hereinafter) or a laser printer that prints out data from an image data supplying apparatus, such as a personal computer (called PC hereinafter) and a digital camera, on regular sheets or OHP sheets. Known single-beam exposing system has gradually been replaced with multi-beam exposing system for enhancing image-forming speed in this type of image forming apparatus.
The multi-beam exposing system enhances an image-forming speed by simultaneous scanning several lines with laser beams. Such multi-beam exposing system is proposed, for example, in Japanese Un-examined Publication NO. 9-258125 disclosing digital beam-positional control, Japanese Un-examined Publication NO. 9-314901 disclosing beam-positional control in main scanning direction, Japanese Un-examined Publication NO. 10-76704 disclosing ladder-pattern sensors, Japanese Un-examined Publication NO. 10-142535 disclosing techniques for eliminating operational amplifier-offsetting, Japanese Un-examined Publication NO. 11-84280 disclosing power control, Japanese Un-examined Publication NO. 11-951432 disclosing several resolutions, Japanese Un-examined Publication NO. 11-202229 (Japanese Application NO. 10-6776) disclosing beam-positional control in main scanning direction, and Japanese Un-examined Publication NO. 10-356022 disclosing relative beam-positional control in sub scanning direction. These proposals mainly relate to light exposed position control. Particularly, Japanese Un-examined Publication NO. 11-202229 discloses precise beam-positional control in main scanning direction used in light beam scanning apparatus and image forming apparatus.
The known image forming apparatus, however, has the following drawbacks:
Light beams used for control are usually detected at edges on an area to be scanned due to optical system structure. The known image forming apparatus cannot generate constant light-beam shape or power on this area. Inconstant light-beam shape or power causes inaccurate detection of each beam position, thus resulting in inaccurate light beam control. Such problem mostly occur in using a compact polygon mirror having a small reflecting surface for light beam scanning for higher beam scanning speed. A compact and thus light polygon mirror relieves its load for increasing rotation number.
Moreover, difference in wavelength or power of several laser sources causes a problem in that a scanning-finishing point fluctuates even though each beam scanning-starting point is precisely controlled. In other words, it causes fluctuation in image-forming area, or image magnification for each beam due to difference in reflectivity of lenses or glasses caused by difference in wavelength over several light beams.
Furthermore, monochrome-color switching machines have a problem of increase in toner consumption in color registration control.
In order to overcome the foregoing problems for known image forming apparatus, a first purpose of the present invention is to decrease in toner consumption in color registration control for a color image forming apparatus, in detail, to attain high color overlapping accuracy by color registration control with minimum test pattern generation, followed by beam-positional control.
A second purpose of the present invention is to provide an image forming apparatus for efficient and rapid monochrome and color image forming. In other words, it is the purpose to solve problems on two types of color image forming apparatus most popular at present, thus providing an image forming apparatus with high color image-forming speed and much higher monochrome image-forming speed.
A third purpose of the present invention is to attain higher image quality in high-speed image-forming apparatus using multi-beam scanning system. An image forming apparatus with high beam positional-detection accuracy even at edges on an area to be scanned on which light beam shape or power tend to fluctuate enhances beam positional-control accuracy. Moreover, an image-forming apparatus having a detector for detecting an image-forming area (image magnification) for each beam to detect image magnification for each light beam provides image-forming areas (image magnification) matched for several light beams for forming high image of quality irrespect of difference in wavelength of light beams.
In order to attain the first purpose, the image forming apparatus according to the present invention maintains a constant beam scanning state so that light beams scan each drum always in the same state when a desired color-overlapping accuracy is obtained by one-time color registration control, thus achieving constantly fine color overlapping without generating a test patterns several times or consuming toner unnecessarily. The beam scanning state is monitored by sensors provided in the vicinity of a photosensitive drum.
An optimum scanning state may be different for each light beam in a high-speed monochrome image formation mode and a color image formation mode. A control state for each light beam in each mode is stored in memory for reproduction of the same state without performing again color registration control when the monochrome image formation mode is switched to the color image formation mode, thus consuming no toner unnecessarily and also shortening a waiting time for image formation when the mode is switched.
In order to attain the second purpose, the image forming apparatus according to the present invention achieves efficient and high-speed color image formation based on 4-tandem system in combination of multi-beam exposure technique. Color images are formed by scanning 4-color drums with four light beams like a conventional 4-tandem system at a speed the same as the conventional system. On the other hand, monochrome images are formed with light beams including beams which usually used for colors other than monochrome image formation. For example, in formation of K-image only, light beams used for Y-, M- and C-image formation are all used for the K-image, thus achieving four-fold image formation performance. For such image formation, a transfer speed of each photosensitive drum and transfer section is adjusted to match an image-recording speed for high-speed operation with optimum control of other requirements.
Accordingly, the present invention is provided with a beam-passage switch in the laser scanning system for operating the apparatus efficiently. Moreover, sensors are provided in the vicinity of the photosensitive drum to determine whether each light beam has scanned a certain portion with no inclination or a certain scanning area (range), or whether each image magnification has provided a desired size, and also a control mechanism is provided for precisely scanning each photosensitive drum with each light beam based on information always correct when detected.
In order to attain the third purpose, the present invention provides an image forming apparatus for accurately detecting beam positions in a light beam scanning direction also accurately detecting image magnification. In general, sensors for detecting scanning light beams output different response waveforms depending on beam shape or power at edges of a scanning area at which beam shape and power relatively fluctuate, which provides incorrect information. The present invention therefore obtains a beam center position from information on a point at which sensors start to respond and another point at which they cease responding and, based on the center positional information, performs several types of control. This achieves several types of accurate control based on correct beam positional information even at the edges of scanning area at which beam size and power fluctuate, thus forming images at high accuracy. In other words, the image forming area is shifted to detect a point at which the sensors start to respond and another point at which the sensors cease responding for each light beam and, based on this information, define an image forming area for each light beam, thus forming images at high accuracy even though beam size and power are different over the light beams.
Against a problem of difference in image forming area, the present invention is provided with a detector to detect an image forming area (image magnification) for each light beam for image forming area setting for each light beam so that the centers of image forming areas of light beams meet each other for minimizing the effects of difference in detected image magnifications for the light beams. Moreover, adjustments to the clock frequency which is the basis for defining the image forming area for each light beam allows the image forming areas for the light beams to meet each other precisely. The present invention also provides selection of an optimum clock frequency as the basis for defining the image forming area for each light beam from among several clocks.